DE 4 101 290 and DE 19 831 574 describe textile surface heating elements having at least two opposed strips made of electrically conductive, non-insulated fibers or wires between which there extends a plurality of non-insulated heating conductors consisting of an electric resistance material.
This type of heating element is primarily used for heating seats in motor vehicles. Because of chemical and mechanical influences such as water, salt and movement on the seats, both the heating conductors and the contact conductors are attacked and locally destroyed in the course of time.
If part of the contact conductors are destroyed, a transition resistance occurs which generates heat along the heating strips which may lead to impermissible heating of the heating element.
DE 4 101 290 and G 90 075 19 disclose a heating element whose mechanical service life is intended to be prolonged by contact strips extending in a wave-like fashion. This arrangement reduces the problem of overheating, but does not eliminate it. To prevent corrosive attacks on metallic conductors, there are known surface coatings made of silver and tin, but they can be destroyed by electro-corrosion if salt and moisture are present in the presence of an electric voltage.
The destruction mechanism develops in such a way that either the contact conductors break as a result of constant bending and movement, as a result of which the cross-section of the contact conductors is tapered in the bent region, or the contact conductors are chemically attacked causing decomposition, or the transition resistance of the heating conductor acting on the contact conductors is increased, so that heating is increased. The installation of temperature sensors can not prevent the thermal problems affecting the contact conductors.
It is therefore the object of the present invention to develop a monitoring device of the above-mentioned type which prevents the heating conductors from becoming so hot in the vicinity of the contact conductors that the permissible thermal limit temperature of the surrounding materials is exceeded.